Some embodiments are directed to a magnetoresistive device, and in particular, relates to a non-collinear magnetoresistive device that has non-collinear magnetization configuration.
A magnetoresistive device (hereinafter simply termed an MR device) fundamentally has a three-layered structure, which is constituted with a free layer, a fixed layer, and a non-magnetic layer disposed between those two layers. From the viewpoint of the principle on which MR devices work, there are two types of MR devices: one that exploits the giant magnetoresistance effect (GMR: Giant Magnetoresistance Effect), and the other that exploits the tunnel magnetoresistance effect (TMR: Tunnel Magnetoresistance Effect). With both of these types of MR devices, the orientations of magnetization of the free layer and of the fixed layer being parallel or being anti-parallel, corresponds to either of “0” and “1” of a digital signal. It is expected that MR devices will be implemented in practice as storage cells or unit cells in an MRAM (magnetoresistive random access memory) which will be an alternative to the DRAMs and SRAMs that are currently in wide use. Since MR devices can be miniaturized, it is expected that they will be put into practical use as memory cells for gigabit class non-volatile memories.
Writing information into an MR device is performed by applying a current (i.e. a current pulse) in the direction perpendicular to the three layers of the MR device. When current is applied to the MR device in this manner, spin transfer torque (STT) acts upon the free layer. Depending on the direction in which the current flows, the magnetization configuration of the free layer and the fixed layer is changed from parallel configuration to anti-parallel configuration, or conversely is changed from anti-parallel configuration to parallel configuration. Reading out information from the MR device is performed by utilizing the fact that there is a difference in the magnitude of the magnetic resistance (MR) between the free layer and the fixed layer depending on the magnetization configuration of the free layer and the fixed layer is in the parallel configuration or in the anti-parallel configuration (i.e., the magnetoresistance effect). When a current is applied so as to flow in the direction perpendicular to the MR device, the voltage between the free layer and the fixed layer differs depending on whether the magnetization configuration of the free layer and the fixed layer is in the parallel configuration or in the anti-parallel configuration.
While, in the initially produced MR devices, an easy magnetization directions of the free layer and the fixed layer were in-plane (in-plane type), in improved MR devices which have been produced thereafter, an easy magnetization directions of the free layer and the fixed layer were in perpendicular to the plane (perpendicular type). In both the in-plane type MR devices and the perpendicular type MR devices, the easy magnetization directions of the free layer and the fixed layer are the same. In other words, if the easy magnetization direction of the free layer is in perpendicular, then the easy magnetization direction of the fixed layer is also in perpendicular. An MR device in which the easy magnetization directions of the free layer and the fixed layer are the same is termed a collinear MR device. In most cases, both the free layer and the fixed layer are each composed of a ferromagnetic material crystal. In that case, the easy magnetization directions of these layers are the same direction as the c-axis of the crystal axis.
However, collinear MR devices generally have a relatively long writing time (switching time). The shortest switching time that has been currently reported is 3 ns (nanoseconds) for practical use MR devices. In view of future developments, in particular, of the application in L1 cache memories (primary cache memories), it may be required for the MR devices to have a switching time as short as 1 ns or shorter.
To solve this problem, there has been proposed a non-collinear MR device that has a magnetization configuration that is not collinear, that is, that has a non-collinear magnetization configuration. The non-collinear MR device has a tilted easy magnetization direction (in the free layer). To achieve such a tilt, it has been proposed that the MR device is produced by using an oblique evaporation method, so that it has a tilted crystal axis (in the fixed layer). However, the oblique evaporation method is a special method, with which it is difficult to control a crystal growth.
Accordingly, it has been proposed to obtain a non-collinear MR device by improving the device construction instead of using the special oblique evaporation method. PTL#1 discloses such a non-collinear MR device, which has biaxial anisotropy, and also discloses a non-collinear MR device having “cone anisotropy” (see, for example, paragraph 041 of PTL#1).